These are stainless steels with main phase that of ferrite (α-Fe) or martensite (phase that results from the rapid cooling of the austenite). They contain 10,5-27% chromium, with very little or none at all nickel (<2%). Some of them have molybdenum or titanium and niobium.
The martensitic stainless steels contain 12-17% chromium and have higher carbon content. These kinds of stainless steels are subjected to specific heat treatment that leads to an increase of their hardness. They are used for the manufacturing of turbine propellers, cutlery, knife blades etc.
The ferritic stainless steels share the same advantages (mechanical properties and corrosion resistance) with the austenitic types. In some cases their performance is superior to that of the austenitics. In short, we mention below the special advantages of the ferritic stainless steels.
- Their magnetism, which may prove useful to several applications.
- Ferritics have lower thermal expansion (they expand less than austenitics when heated) and thus deform less at high temperatures. Moreover, thanks to their lower thermal expansion coefficient, ferritic grades are much less prone than austenitic alloys to high temperature cyclic oxidation scaling.
Ferritics have higher thermal conductivity and consequently they conduct heat more evenly than austenitics (suitable for electric irons, heat exchangers, radiators etc).
- Ferritics stabilized with niobium have excellent creep resistance (they deform less than austenitics in response to long–term stresses).
- Ferritics are easier to cut and work than austenitics (which require special tools and more powerful machines and generate greater tooling wear).
- Ferritics are significantly less prone to springback than austenitics, during cold forming.
- Ferritics have higher yield strength than type 304 austenitics.
- Ferritics, unlike austenitics, are not prone to stress corrosion cracking. Later we make more detailed reference.
Ferritic grades fall into five groups – three families of standard grades and two of "special" grades.
Group 1 (409/410L)
has the lowest chromium content (10-14%) of all stainless steels and is also the least expensive. This group can be ideal for non–or lightly–corrosive environments or applications where slight localized rust is acceptable. Typical applications are automotive exhaust–system silencers for the type 409, while 410L is often used for containers, buses, coaches and LCD monitor frames.
Group 2 (430)
is the most widely used family of ferritic alloys. Having a higher chromium content (14-18%), group 2 grades show greater resistance to corrosion and behave most like austenitic grade 304. Typical uses include washing–machine drums, household utensils, dishwashers, pots and pans and indoor panels. Generally speaking, we note the fact that 430 is suitable to replace 304 in some applications where the use of the latter is judged as an excessive (over-alloyed) and expensive choice (i.e. catering equipment).
Group 3 includes 430Ti, 439, 441.
Compared with group 2, these grades show better weldability and formability. Their behavior is even, in most cases, better than that of 304 austenitic grades. This group has chromium content between 14 and 18% and is characterized by the presence of stabilizing elements like titanium and niobium. Sinks, exchanger tubes, exhaust systems (longer life than with type 409) and catering equipment are some of the typical applications. Group 3 grades can replace type 304 in applications where this grade is an over – specification.
2. Τhe Limited Drawing Ratio (LDR) is an important deep-drawability parameter. It refers to the quotient of the maximum blank diameter (D) that can be deep drawn into a cylinder in one step and the diameter (d) of that cylinder. LDR=D/d.
Group 4 includes types 434, 436, 444, etc.
These grades have added molybdenum, for extra corrosion resistance. The chromium content lies between 14 and 20%. Typical applications include hot water tanks, solar water heaters, visible parts of exhaust systems, electrical – kettle and microwave oven elements, automotive trim and outdoor panels, etc. Type 444’s corrosion resistance level can be similar to that of type 316.
Group 5 (types 446, 445/447 etc)
has additional chromium (18-30%) and contains molybdenum, for extra corrosion and scaling (oxidation) resistance. The grades of this group are superior to type 316 in respect of these properties. Typical uses include applications in coastal and other highly corrosive environments.
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